US3001081A - Photoelectric gage - Google Patents

Description

L Sept. 19, 1961 J. L. BOWER I 3,001,081

PHOTOELECTRIC GAGE Filed Aug. 19. 1957 3 Sheets-Sheet 1 g: l l .J

REcTIFIER 58 I 2 f PASS 57 FILTER 54 53 l 52 f PASS I 82 FILTER INVENTOR.

FIG 5 JOHN L. BOWER ATTORNEY 3 Sheets-Sheet 2 R m wE m m E W m m 5558 m L N E91 .9. I 52 50023 1 on m E0252 0.3. w Emzmm wm z I 2596 in J 5.280 mm mm 8 I IN I I 2 1 F VL wzammfim 2063 mod I $23850 m $3551. on B mm in. l w Emzwm wwsi 5558 mm Em: an mm 3 .m

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ATTORNEY Sept. 19, 1961 Filed Aug. 19, 195'? Sept. 19, 1961 J. 1.. BOWER PHOTOELECTRIC GAGE 3 Sheets-Sheet 3 Filed Aug. 19. 195? ATTORNEY are.

3,001,081 PHQTQELECTRIC GAGE John L. Bower, Downey, (Ialiii, assignor to North American Aviation, Inc. Filed Aug. 19, 1%57, her. No. 678,886 11 Claims. (Cl. 250-222) This invention relates to photoelectric gages and concerns particularly gages of the relatively moving optical grid type.

An object of the invention is to provide a simplified phototube system for a photoelectric gage and to simplify means for producing oppositely phased impulses.

In order to produce highly precise indications of measurements and positioning of machine tools and for precise measurement of long distances as well as short distances, a digital register system is desirable. For producing digital indications, relatively movable grids may be provided as described in my copending application, Serial No. 526,086, filed July 5, 1955. In such a system there are numerous opaque lines ruled on translucent or transparent rods to form grids. In successive relative positions of the grids they permit light to be transmitted from a light source to photoelectric responsive means or out off such light so as to produce electric impulses correspending in number to the distance moved by one grid relative to the other. In order to increase precision, obtain indications of directionality and either register the net movement or totalize the excursions in one direction separately from the excursions in the opposite directions, a plurality of photoelectric tubes may be arranged in bridge connection so as to produce pairs of oppositelyphased impulses which in turn are so arranged as to introduce a quadrature or smaller angular relationship between the sets of oppositely-phased impulses. The photo tube system and associated optical apparatus and some of the electrical elements are of necessity mounted in a gage head.

In order to increase the compactness and lightness of the gage head, it is desirable to simplify the optical system and reduce the number of photoelectric response devices required to achieve precision or to accomplish directional signals.

It is accordingly an object of the invention to effect each phase-opposition by use of a single photosensitive device instead of a pair of such devices.

One of the problems in detection of light variations in each phase of the gage head is the subtraction of two signals representing two transmitted light intensities while providing a sum signal that is adequate to hold constant the gain of the channel, in order to achieve satisfactory functioning of the electrical circuits for converting the light impulses to counter registration.

It is accordingly an object of the invention to satisfy the foregoing requirements as well as to achieve a minimum of drift of the electrical circuits.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

In carrying out the invention in accordance with a pre: ferred form thereof, glow lamps with electrodes which become illuminated or glow only during alternate half cycles of an alternating-current supply source are utilized to provide oppositely-phased signals. Thus the subtraction of two signals is accomplished by the inherent characteristics of the glow lamps to permit a single photo responsive device to indicate subtraction of two signals. Separate optical paths to the photo responsive device are provided for the light emitted from alternately glowing lamp electrodes. Different portions of a relatively movable rod grid and head grid are interposed in the two optical paths.

The electric output from the photoelectric responsive devices is filtered to produce a fundamental frequency signal for actuation of the counter circuits. It is also filtered by a filter cutting off the fundamental for supplying a second-harmonic signal responsive to variations of average light transmission to produce a control signal for regulating the voltage output of the supply source or the gain of an amplifier interposed in the output circuit of the photoelectric response device.

A better understanding of the invention will be afforded by the following detailed description considered in con-' junction with the accompanying drawing in which FIG. 1 is a perspective view of the general type of photoelectric gage in which the invention of this application may be employed;

FIG. 2 is a fragmentary diagram illustrating the arrangement of relatively movable optical grids for producing light impulses as relative movement of the grid takes place;

FIG. 3 is a block diagramillustrating the electrical circuit relationship of the elements of a photoelectric counter system for a gage or machine tool positioner in accordance with the invention;

FIG. 4 is a schematic diagram illustrating the arrange- I ment of different portions of the optical grid, one portion of which is shown in FIG. 2, in order that appropriate phase relations will be obtained for different signals;

tube system employed in the arrangement of FIG. 3;

FIG. 6 is a perspective view of a different form of lamp which may be employed in lieu of the symmetrical electrode type illustrated in FIG. 5

FIG. 7 is a circuit diagram illustrating the connections of light sources of the type of FIG. 6 employed to pro duce the optical effect of the arrangement of FIG. 5;

FIG. 8 is a schematic diagram of the optical system employing lamps of the type illustrated in FIG. 6;

FIG. 9 is a circuit diagram illustrating in detail the circuit arrangement ofa preferred form of photoelectric responsive device such as a photo transistor employed for the sake of compactness and minimum power requirements; and j FIG. 10 is a graph illustrating the phaserelationship between different signals obtained in the arrangement illustrated in FIGS. 5 and 3.

Like reference characters areutilized through the draw-' ing to designate like parts. I

Referring to FIG. 1, there is shown a photoelectric gage having a base 11 upon which is mounted a rod 12 with a gage head 13 movable along the rod 12 in accord ance with a linear measurement to be made or position- 1 ing of a machine tool. There is a register 14 connected to the gage head 13 having an electric cable 15 and having a pair of dials 16 and 17 for recording the summaa plurality of closely spaced lines ruled thereon. It is to be understood that in FIG. 2 the dimensions are greatly exaggerated and that the division lines are actually very closely spaced of the order of of an inch or less apart. The gage head 13 also includes a grid 19 which corresponds to the gage rod 12 having grooves 21 corresponding to the grooves 18of rod 12.

As shown in FIG. 4, preferably the head; grid 19 is divided into 4 separate portions 19a, 19b, 19c and 19d so Patented Sept. 19, 1961 i FIG. 5 is a schematic diagram of one half the photopositioned in relation to the divisions of the grid 12 that When the head grid 19a presents minimum obstruction to the passage of light beams 22, the head grid 1% presents obstruction. to. the passage oflight beams 25. Likewise. thehead grids. 19c. and 19d are..so..located .i-n. relation. to each other that one.;presents. maximum lob; struction to. the passageof lightheams Zdwhenthe other. presents minimumobj ection. to the passage. of lighLheams 25. However, the head grids 19c. and. 19d areso. p osi: tionedinrelation to, the .head ,gridslfic. and 1% that they arein anintermediate position when thehead grids. 19a

and 1% are in the relatively oppositepositions. Employ ing theterminology of space-phaserelationship,.it may be saidthat theheads grids 19a andlfi'b. are. positionedin. opposed-space phase; likewisethe. head grids19c and.v [9d are positioned in opposedrspace phase with the head grids 19c and 19.5! in quadrature space phase relationship to head grids 19a and 19b.

.As illustrated irrFiG. 3, in order. to. convert the light impulses resulting from relativev movement of the. girds. l2; and 19 into, suitable electricalsignals for directional indication and registration of movement on counters, a; system of electrically connected. elements iscmployed. There, is a phototube system. 26; preferably dualior. the; sake of. increasedprecision and. directionality as.indicated.v by the division line 27 to representduplicate elements... Provided also are a phase sensitive. --demodulator.; lfitfor one;half. of the phototube. system, a.s.econd.pl1ase:;sensi= tiveedemodulator 2.9 for the second halfof'the-phototube: system if a second half is employed, and. countersfilrand; 32 for indicating left and right and excursions respefiivee ly.' Asingle reversing counterfifimay bevemployedifor indicating position at anyi-nstant. Acomrnon energizing and phase-reference source of alternating current .3L4'is provided having output terminalsldl.

Flip flops or equivalent bistabledevices-35- and;36:areprovidedfor converting the phase=responsiveoutputs-signals of the. demodulators 28 and 29 into. appropriate.

steady state;voltages,, and a. logicalnetworktfi is: em: ployed for interpreting the state and:the;change:in..state of the flip fiops 3'5 andfififto produce the rcquisitmactuaetions ofeither the left counter 31hr the. right counter-33, andihe: reversing counterfih to; register-the sdesiredzmeas urcment.

BIG, representsaphototube system'which; mayconstitute one half of the system 26 illustrated in FIG-.1 3; A. lamp3$ is provided, which is of suchatypeas to become illuminated only on alternate half cyclesof analternating-current source 34 or to become illuminated in: differ ent, areas during alternate half cycles. the lamp 38 may be of the gaseous discharge-typehaving azpair; of. symmetrically mounted electrodes 3.9 and 40 such. as. in certain types of neon tubes with aglow dis ch'arge' appearing upon the surfaces of the electrodes 39 and 40 alternately in alternate half cycles. When thistype of neon tube 38 is employed, optical directing' meanssuch as condensing or focusing lenses 41, 42, 43 and mirrors 44, 45, 46 and 47 and 48 are provided to direct the light from the lamp 38 in two separate optical paths 22 and 23 upona photo responsive device 51 such as-a photo transistor, for example;

The optical paths 22 and 23are so arranged that dilie'r ent portions of the rod grid 12 and "the head grid 1 9 are interposed in the light paths22 and 23;

As represented schematically in FIG'. 5; an amplifier- 52 is" provided for converting output electrical'impuises-iof the photo responsive deviees si into electrical signal's of.

sufficient amplitude in a counteractuating input channel schematically represented by a line 53' corresponding to conductors 55 and 56-, for example, ofFIG. 3'. A suit: able filter 54 is interposed in the channel .53 having a. cutoff point for transmitting only the fundamental frequency of the alternating-current source or generator 34;

The amplifier 52is provided w-it-hagain control tenni For example,

nal 57 which is inversely responsive to magnitude of unidirectional voltage to enable the gain to be adjusted'automatically for constant output level in the channel 53.

To this end a filter 58 and a rectifier 5'9 are interposed between the output of thccampli er 52 and the voltage responsive gain control terminal 57. The filter 58 has a cut-off frequency below the; second harmonic of thealternati-ng current; source. 34 and is-- preferably a second harmonic bandpassfilterr However; the invention is not limited to the specific output level maintenance arrangementudescribedrand.doesnot exclude the use .ofan oscillator. or. generator .3.4. havinga voltage-responsiveoutput: voltage regulating terminal connectedto the output of the rectifier 59.

If. desired, in. order. to. simplify the optical, arrange ments, two. separate lamps instead. ofthe symmetrical. electrode lamp 38. may beemployed, such as neonlamps oftheGeneralElectric NET-1.7 type, for example, as il'lns.-.-

tratedinFIG. 6.,having a. center. electrode 61v anda COIlr: centricring electrodedz with. a luminousglow appearing; on tliecenter'electrode 6.1.for every other cycle ofitheexciting alternating current. In. order. that the center. electrodes 61. of, complementary tubes. willv glow: alter:- nately, such. tubcstare connected as. illustrated. in. FIG). 7. 'withthe. center electrode .61 .of. one neontube. connected; to. the,.conductor 63 of :the; alternating 7 current source. and. theother, center. electrode 61 connected .to the other:-

' conductor 6.4-.of;the; alternating current source3.4

1 of,a.voltagedividerconsisting of resistors 71 and.72 CD112.

nectedbetween. a, point. of suitable negative voltagesunh; as. at.-1.1..vo.1t. directrcurrent terminal73 and. a,zer o:voltr or ground terminal 74. The photo transistor s-lrlike-o wisehas. anemitter. 75. connected through. a. resistor.7.6 to the;zer.o potentialterminal 74 anda collector '77 com-- nectcd.through..a:rcsistor '78 to. the negative bias terminal. 73,. Ereferably a. relatively large capacity lay-passcon; denser 79 is connected across the resistor 76.and,a.rela-, tivelv,smal l.v capacity coupling condenser 81 is connected betweenthe, collector .77 andan output terminal 82,.serv ing as an input terminal .for,the amplifier-52 or EIG.. 5;.

Since the two electrodes 39. and 40 of, FIG. 5 8 .9w separately at opposite half cycles, When-the'light. from. the-.twoofithemiscombinedin the sensitive area .of.thc photo. resppnsivedevice .51, the resulting current in .the." two. half cycles. will produce fundamental components.- that oppose each, other in phase. In effect therefore the fundamental frequency components subtract in, order toperform the subtraction required in the type of gage head illustrated in FIG. 1 for the system represented. schematically in FIGS..2, 3 and 4. At the same timethe secondharmonic currentsproducedin the separate half cycles. are additive in. the. double frequency pass. filter 58'. The rectifier 59 produces a current proportional. to,tl1e average.transmissionthrough,the two optical channels..22 and 2,3..o.f the systemforonephase of the gage; head in. order, to. supply the requisite control voltage to: theterminal 5.7 formaintaining uniform gainof the channel53.

In. the. arrangementillustrated. in FIG.. 5, the light; beams. 22. and. 235 from .the,tw.o electrodes 39 and. 40. of the neon. lamp 38'. are... separately picked. up and re: fiectedin, mirrors 44 and 45, transmitted through the grid; or gage. rod 12 through the Windows or portions 19a.- and 19b of the head grid 19 to mirrors 46, 47 and 48 and through a condensing lens 43 to the photo. sensi, tive surface of the device 5-1. It is to be understood that;

the mirrors 48 are tilted to give the best superposition of the images on the photo sensitive surface. The amplitude and direction of the signal from the photo sensitive device 51 appearing on the terminal 82 indicate the degree and direction of predominance of transmission through the grid portions 1% and 19b.

The same type of action takes place when employing the lamp arrangement of FIGS. 6, 7 and 8 with the beam from one lamp passing through lens 65 and the grating portions 19a and the beam from the other lamp passing through the lenses 66 and the grating portion 19d.

The operation is explained analytically by the curves of FIG. in which curve A represents the output of the second harmonic pass filter 58 similar to the output from a full wave unfiltered rectifier.

When the beam in the light path 23 is interrupted, the output voltage appears as represented in curve B of FIG. 10, and is similar to the output of a half wave slightly filtered rectifier. On the other hand, when the beam in the other light path 22 is interrupted, the output appears as shown in curve C of FIG. 10. It is similar to the output of a half wave slightly filtered rectifier excited by a voltage 180 out of phase with the excitation for the electrode 39 of the lamp 38.

Similar curves D, E and F of FIG. 10 represent the output of the other half of the phototube system 26 of FIG. 3, similar to that illustrated in FIG. 5, except with .the light paths 24 and 25 so interrupted by the quadrature displacement of the head grid portions 19 or 19d that the curves E and F are in quadrature relationship .to the curves B and C, respectively, of FIG. l0, curves .A and D in effect being in phase opposition.

Although the invention has been described and'illus :trated in detail, it is to be clearly understood that the :same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of .the appended claims.

I claim:

1. In a photo electric gage, a phototube system comprising a pair of terminals for connection to a source of alternating current, gaseous discharge glow lamp means having opposite electrodes operatively connected to said terminals to become luminous on alternate cycles of alternating current supplied to such terminals, a gage head grid and a rod grid, relatively movable, a photoelectric responsive device, optical light occulting means mounted to form two independent paths between said photoelectric responsive device and said lamp means with one electrode included in one path and an opposite electrode included in the other path, difierent portions of said grids being interposed in said optical paths for occulting light in one or the other of the optical paths according to the relative position of the grids, a counter system input line including a phase-sensitive demodulator operatively connected to said photoelectric response device for receiving fundamental frequency pulses in response to relative movement of said grids, a fundamental frequency filter interposed in said counter system input line, a voltage-responsive control terminal for controlling output to said counter system input line, and a filter having a pass band at twice fundamental frequency, interconnecting said control terminal and said counter system input line for compensating variations in light transmission to maintain substantially constant output to said counter system input line.

2. In a photoelectric gage, a phototube system comprising in combination a pair of terminals for connection to a source of alternating current, a glow lamp having a pair of symmetrical electrodes operatively connected to said terminals to become luminous on alternate half cycles of alternating current supplied to said terminals, a gage head grid and a rod grid relatively movable, a photoelectric responsive device, an optical light directing means mounted to form two optical paths between said photoelectric responsive device and respectiveelectrode's of said glow lamp, different portions of said grids being interposed in said optical paths for occulting light in one or the other of the optical paths according to the relative positions of the grids, a counter system input line including a phase-sensitive demodulator operatively connected to said photoelectric device for receiving pulses of fundamental frequency corresponding to the source of alternating current connected to said terminals in response to relative movement of said grids, a fundamental frequency filter interposed in said counter system input line, a voltage-responsive control terminal for controlling output to said counter system input line, and a filter having a pass band at twice fundamental frequency interconnecting said control terminal andsaid counter system input line for compensating variations in light transmission to maintain substantially constant output to si-ad counter system input line.

3. A phototube system as in claim 1 wherein an adjustable voltage source of alternating current is connected to said terminals, said voltage source having a voltage control terminal constituting the voltage responsive control terminal.

4. A phototube system as in claim 1 wherein an amplifier is interposed between the photoelectric response device and said filters, said amplifier having a gain control terminal constituting the voltage-responsive control terminal.

5. A photoelectric gage comprising'rneans for alternately producing one and then the other of two light' beams, grid means having relatively moving elements positioned to receive light from said light beams for modulating the light from said beams in opposite phase relationship to each other, said grid means having a first portion positioned to receive light from one of said beams and a second oppositely phase related portion positioned to receive light from the other of said beams, means for producing a signal representing the diiference between the light passed from said first and second oppositely phased grid portions, said dilference producing means comprising a single photosensitive responser positioned to receive the light passed from both said first and second grid portions, said responser having an electrical output proportional to the difference between the light received thereby from said first and second beams.

6. In a photoelectric gage, a phototube system comprising a pair of terminals for connection to an alternating-current source, a pair of gaseous discharge glow lamp means of the concentric electrode type with a center electrode and a circular surrounding electrode, said electrodes being operatively connected to said terminals, the center electrode of one lamp connected to one terminal and the center electrode of the other lamp connected to the other terminal, a gage head grid and a rod grid relatively movable, a photoelectric response device, optical means for focusing an image of the center electrode of each lamp upon the photoelectric responsive device including light directing means forming separate optical.

paths between said photoelectric response device and said center electrodes, different portions of said grids being interposed in said optical paths for occulting light in one or the other of the optical paths according to the relative positions of the grids, a counter system input line operatively connected to said photoelectric response device for receiving fundamental frequency pulses in response to relative movement of said grids, a fundamental frequency filter and a phase-sensitive demodulator interposed in said counter system input line, a voltage-responsive control terminal for controlling output to said counter system input line, and a filter having a pass band at twice fundamental frequency interconnecting said control terminal and said counter system input line for compensating variations in light transmission to maintain sub- '3. stantiallw constant; output: to; said. counter.- system; input 71 In ai.photoelcctric; age;. a phototubei'systemroemeprisinga pain-of ,.terminals =,fonconnection, toa, source, of alternatingicurrent, glow lamp means having different elecimdes...operatively connected to,said terminals totbecomeluminouspm alternate; cycles of alternatingcurrent supplied,to;such,fterrninals, aphotoelectric responsive de-, vice shaving a,singlephotosensitiveresponser, a gage head, gtid:.aud; arod,grid, relatively movable, forming optical liglitxoeculting meansrmounted to. form tWo independent paths between. said photosensitive, response: and said larn xmeans with. oneielectrode' included in one path, and, another; electrode included in the other path, diiferent PQIlZlODSiOf. said grids being interposed. in said optical paths-,rforocculating light intone or the other of the optical .paths according t0.the= relative position of the grids, and a counter system input line including a, phase-sensitivei demodulator operatively connected to said photoelectricresponse-device for receiving the output of said photosensitive -responser in; response, to relative rnovementof. saidgrids,

8. In a photoelectric gage, a phototuibe;systemv com prising, incombination a, pair, of terminals'for connection tosazsouree; ofz. alternating current, glow lamp means, having-electrodes operatively connected tosaid terminals. tohecome-lumlnouson alternate halfcycles of alternating; current supplied to said terminals, a gage head grid and. a rod, gridzrelatively movable, a photoelectric responsive device,. an; optical. light directing meansmonnted to form wo. opticaLpaths-y between said photoelectric responsive devicesand respective alternately luminous electrodes of said glow lamp means, difierent' portions of said, grids being interposedin said optical paths for occulting light in; one, or; the other of the optical paths according to therelativeapositionsof the grids, a counter system input lineeincluding a phase-sensitive demodulator operatively connected tcnsaid photoelectric device for receiving pulsesgin. response to relative movement ofsaid grids, a voltagerresponsivecontrol terminal for controlling output-:to, said countersystem input line, and a filter having a: pass; band, at twice fundamental frequency interconnesting, said, control terminal wd said counter system input'line for-:compensating variations in light transmissigunato, maintain; substantially constant output to said counter system input line.

9; In a-photoelectric gage, a phototube system comprising a pair of terminals for connection to an alternatingcurrent: source, lamp means having alternately luminous elements, said lamp means being operatively connected togsaid terminals, 21 gage head grid and a rod grid, relatively movable, aphotoelectric response device having one-photosensitive responser light directing means forming two separate opti"al paths between said photosensitlve--responser and said lamp elements, one of said lamp elements being included in one of said paths, the other of said, lamp elements being included in the other of said paths, different portions of said grids being interposed in said optical paths for occulting light in one or the other of the optical paths according to the relative positions of the-grids, and a counter system input line including phasesensitive demodulator means operatively connected to saidaphotaelestricirespcnsived v cetforrecerr ne sa uteput-,ofisaid: photosonsitiye;responser;inrespons M relate iv zmv mcntctrsaidcsu t 10 In combinaticn i phot lectric ys em omp ising. a pair; of. terminals for; conncctionlto, avsoprce otdalter hating current, lampzngeans operatively-connected' toward;- tenrninals-having, elements: whichgbecoinc; luminou on alternate cycles of, altcrna tirig current supplied to; saidti: lamp n eans, ayphotoelectric; responsive: device havinga; singles, photosensitive responser, optical light; occultiuge means having relatively movable.menibersrincludingwore tions, displacedin space :phase mourned to ,fornniuo independentnpaths betweensaidjphotosensitive responsenand-i, saidrlamprmeansgwith; one lampselement. includedinaonae path and another lampeelemejntr includedzin the others: path,. difierent portions .of .s aid; cccultingxmeansbeing-ainterposed; in saidopticaLpaths :r'or: occultingslightrimonce or=thezotheriof theopticall paths: according -;to;.the:relative position. v0:5: thev grids; and producing oppositely,- phasede'; impulses, and azcountersystem: inputyline including E1L phase sensitive; demodulator. for:- converting: the impu-lsess intogsignals-rof GPPOSllQrPOlfiIi'LY operatively connected toe said photoelectric response v device for-receiving the: oute put of said photosensitive responser-inuresponse to-rela-z. live-movement of said'light occulting means.

11; In; a; photoelectric gage, a= photoelectric, system: comprisinga pair of terminals:forccnnectionto antak-l-i ternating-current source, lamp meansv having: alternately-1 luminous elements, said-J lamps means. beingoperativelgtr connected to saidterminals, a gage head grid and a; rod grid; relatively-"movable, a photoelectric response devices having at least one-photosensitive responser, light direct-aingjrneans forming separate optical pathsr-between said photosensitive rcsponser and said'iampelements; dlfil'r' ent portions of said grids beinginter-posedinsaid optical; paths and being displacedin space phase for occulfingg light-in one or the other optic'al paths according tonthe relative positions of the grids, and producing oppositel phased impulses in response to relative grid movement, and'a' counter system input line, including phase-sensitive demodulator means,- ior converting the impulses; into-- signals of opposite-polarity, operativeiy connected" to saidiphotoelectric-response-device for receiving signals-- in response to relative movementofsaid grids:

References Cited inthe file of this patent- UNITED STATES; PATENTS 2,065,421 Bernardc, Dec. 22,."1'936 2,176,442 Wise Oct, 17, 1939" 2,451,972,. Bowers v Oct. 19, 1948? 2,557,219 Flint et al. June-19, .1951. 2,562,181 Frommer; .July 31, 195.1, 2,586,540- HOlden- Feb. 19, 1952,. 2,656,106 Stables", Oct. 20, 1 953? 2,857,802 Cail Oct. 28,- 1958; 2,861,345 Spencer NOV.'25, 19.58;; 2,880,512 Fenemoreet al Apr; 7, 1959, 2,886,717 Williamsonet al; May 12, 1959 2,886,718 Shepherd et a1 May 12, 1959" 2,916,826 Bower et a1 Dec. 15, 1959

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